Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
400 AM EDT Sat Mar 30 2024

Valid 12Z Sat Mar 30 2024 - 12Z Tue Apr 02 2024

...The West...
Days 1-3...

Deep, closed upper low currently centered off the central
California coast early this morning will continue to track
southeast, reaching the SoCal coast by early Sunday morning.
Southwesterly flow ahead of this feature will bring a moisture
plume into the region, spreading across the Sierra, Great Basin,
and eastward into the UT and western Wyoming ranges. The moisture
will also intersect the SoCal ranges. Slight to near moderate
levels of IVT (up to 300 kg/m/s) and PW anomalies reaching +3 will
help contribute to the higher snowfall accumulations that are
mainly confined to elevations above 5000 ft for CA and nearing 7000
ft for the Uintas in UT. The latest WPC snow probabilities are
high for at least 6 inches in these areas (diminishing in the
Sierra as the event subsides). At the 12 inch threshold,
probabilities are high (>70%) for the peaks of the Transverse and
Peninsular ranges as well as the southern UT and Uintas.

For Days 2-3, a lobe of vorticity digging southward from the PacNW
will absorb with the opening upper low over CA to become a large,
messy trough over the Four Corners region by early Monday. The much
lower heights, anomalous moisture, and orographic flow is likely to
support heavy snow over a good portion of the interior West, Great
Basin into portions of the Rockies. From the higher terrain in
central NV through the western WY ranges and as far south as the
San Juans and along the Mogollon Rim, 6 inch snowfall
probabilities are at least moderate and locally high for the higher
peaks, generally above 7000 ft.

By Day 3, the system kicks to the east, with the best forcing and
lift focused on the CO Rockies and into the terrain of AZ/NM where
6 inch snow probabilities are moderate (40-60 percent),
particularly for the CO Rockies.

...Upper Midwest...
Day 1....

A passing wave of low pressure early this morning will bring a
quick shot of wintry precipitation to portions of northern
Wisconsin and the U.P. of Michigan today. A residual cold airmass
settled over the region combined with moist isentropic lift will
bring a short period of accumulating snow, mainly through this
morning. WPC snow probabilities are low for an additional 2 inches
(up to near 30 percent). A shallow warm nose at around 850 mb may
allow for a brief period of freezing rain, where a few hundredths
of accumulation will be possible. The bulk of the winter
precipitation threat should diminish by this afternoon as the
system passes to the east.


...Northern/Central Plains...
Days 2-3...

An amplified but positively tilted trough across the Intermountain
West will begin to pivot eastward as it evolves into a closed low
centered near the Central High Plains by late Monday. This will
drive pronounced height falls eastward, which will combine with
downstream mid-level divergence, the LFQ of a strengthening
subtropical jet streak, and persistent troughing in the lee of the
Rockies to drive cyclogenesis east of the Front Range Sunday
evening. While there remains some longitudinal spread in the
developing low placement, it is likely to deepen considerably once
it initiates in response to the impressive synoptic lift, and move
east/southeast towards the Southern Plains during Monday.

This deepening low and associated strong synoptic ascent will work
across an environment with plentiful moisture as SW flow around the
longwave trough surges IVT as high as +4 sigma according to NAEFS
into
the Four Corners, which will combine with increasing moist
isentropic lift downstream of the low from the Gulf of Mexico to
produce an expanding shield of precipitation. The column north of
this low is likely to be initially marginal for snowfall outside of
the higher terrain, but as the low deepens and cold air drops south
beneath a Canadian high pressure, wintry precipitation in the form
of heavy snow may expand across the High Plains from WY into the
Dakotas and Nebraska. Here, the latest WPC snow probabilities are
slight to moderate (20-40 percent) for the far northern NE and
western/south-central SD areas. For the Black Hills, 4 inch
probabilities peak at 50 percent.

By Day 3, greater uncertainty exists as the low tracks further east
into the Plains and Midwest. This is mainly tied to the strength
and amplitude of the shortwave energy and deepening low pressure.
The GFS continues to show a stronger/deeper system, allowing for
banded precipitation to linger through the end of Day 3 and tapping
into an increasingly colder airmass while other pieces of guidance
is a weaker/flatter system (and further south) that lacks the cold
air and deep lift to support accumulating snow. Overall it's a
lower confidence setup given the marginal thermal profiles but if
the stronger/deeper solution materializes, dynamic cooling may
result in banded snow northwest of the low that could be wet and
heavy. WPC snow probabilities drop off considerably for Day 3 given
this uncertainty, generally under 20 percent for 2 inches for most
areas.

The probability of significant icing is less than 10 percent
across the CONUS.


Weiss/Fracasso/Taylor